In many processes, for instance in the field of industrial process engineering, chemistry or machine construction, a gas mass, especially an air mass, has to be supplied in a specific manner. Among these are, in particular, combustion processes, which run under regulated conditions. An important example in this context is the combustion of fuel in internal combustion engines of motor vehicles, especially those having subsequent catalytic exhaust purification. Various types of sensors are used to measure the air-mass throughput.
One sensor type from the related art is what is generally referred to as a hot-film air mass sensor (HFM), which is discussed in one specific embodiment in DE 196 01 791 A1, for example. A sensor chip, which has a thin sensor diaphragm, e.g., a silicon sensor chip, is generally utilized in such hot-film air mass meters. At least one thermal resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors), is typically situated on the sensor diaphragm. An air flow that is routed across the diaphragm causes a change in the temperature distribution pattern, which in turn is detectable by the temperature measuring resistors and is able to be analyzed with the aid of a control and evaluation circuit. For instance, an air mass flow is able to be determined from a difference in resistance of the temperature measuring resistors. Several other variations of this sensor type are known from the related art.
One problem with such a type of sensor known from DE 101 11 840 C2, for instance, is that contamination of the sensor type can often occur, such as contamination by oil, other fluids, or other types of soiling. As a rule, the sensor chip is used directly in the induction tract of the internal combustion engine or in a bypass to the induction tract of the internal combustion engine. During operation of the internal combustion engine oil may deposit on the sensor chip and on the sensor diaphragm, in particular. This oil deposit can lead to an undesired effect on the measuring signal of the sensor chip, especially since an oil film on the surface of the sensor chip affects the thermal conductivity of the surface, which results in a falsification of the measuring signals. Furthermore, the oil contamination can also occur during or shortly after deactivation of the internal combustion engine, e.g., a diesel engine.
This is the case especially when, following the deactivation of the internal combustion engine, an overpressure present in a crankcase is reduced via a crankcase ventilation into the induction tract of the internal combustion engine (and thus, e.g., also into the bypass canal of the hot-film air mass meter). Oil vapor or oil mist is often carried along in the process. Therefore, DE 101 11 840 C2 proposes a method for avoiding contamination on a sensor chip with the aid of a supplementary heater. The sensor chip has a sensor region and also a supplementary heater disposed outside the sensor region. This supplementary heater is heated electrically, in such a way that thermo-gradient turbulence occurs in the region of the supplementary heater, which results in deposits of the contamination of the flowing medium in the region of the supplementary heater, beyond the area of the sensor region.
In practice, however, the system disclosed in DE 101 11 840 C2 and the disclosed method have disadvantages in different operating modes of the internal combustion engine. For instance, one disadvantage is that a localization of the thermo-gradient turbulence as intended by the device disclosed in DE 101 11 840 C2 is virtually impossible in practice. Due to the high thermal conductivity of the silicon, the heat generated by the supplementary heater easily moves across the entire chip, which results in a “smeared” temperature distribution and thus to heating of the entire chip.
The problem of contamination of the diaphragm or the sensor surface is made worse by thermodynamic effects. It is known, for instance, that fluid droplets with a gradient in their surface tension are subjected to a force in the direction of the greater surface tension. This usually leads to a movement of the droplet from a lower to a higher surface tension. In particular, this gradient may be caused by a temperature gradient on a surface on which the fluid droplet is situated. The temperature gradient usually shifts from a warmer region of the surface to a colder region of the surface. This effect is discussed in, for instance, V. G. Levich, “Physicochemical Hydrodynamics”, Prentice-Hall, N.J., 1962, p. 373 and p. 380.
As described above, typical hot-film air mass meters are configured such that they have a sensor diaphragm (e.g., a silicon diaphragm) having low thermal conductivity, and a surrounding chip mainland. During operation of the hot-film air mass meter, a temperature gradient and a corresponding fluid wall therefore normally build up at the edges of the sensor diaphragm, i.e., at the border to the surrounding chip mainland. This fluid wall may be fully or partially carried along by the air flow, so that oil droplets end up on the sensor diaphragm and may affect the measurement there. Furthermore, the fluid wall causes an increase in the thermal conductivity at the edge of the sensor diaphragm, which may lead to falsification and drift of the measuring signal.